The present invention relates to a novel process for the preparation of lithium hexafluoro arsenate (LiAsF6) useful as an electrolyte in nonaqueous solid state and polymer electrolyte lithium based primary and secondary batteries.
Lithium hexafluoro arsenate (LiAsF6) is useful as an electrochemical energy producing electrolyte cell with small cation and large anion which never helps good conductivity in non conventional media like non-aqueous solvents, solid state ionic materials and polymer based electrolyte medium. The general method, available in literature deals with AsF5, F2 etc., which are difficult to handle. A few chemicals of the above type need sophisticated equipment to handle, operate and installation needs heavy investment. Apart from that environmentally such unfriendly chemicals need protection.
A literature survey revealed the following references:
1. Fluorine Chemistry Vol II J. H. Simons Ed. Academy Press New York, pg. 38, 1955.
2. Fluorine Chemistry Vol H. J. Emedius Ed. Academic Press New York page 76 (1955).
3. Fluorine Chemistry Vol II W. Large Ed. Academic Press New York page 12(1955).
4. P. L. Soni. Textbook of Inorganic Chemistry S. Chand and Co. Delhi page 425 (1985).
Lithium hexafluro arsenate (LiAsF6) is prepared by all solid state thermal or partial solid state reactions where arsenous trioxide (As2O3) is allowed to react with lithium hydroxide or lithium oxide or lithium carbonate or lithium nitrate in equimolar proportions under controlled thermal conditions so as to get lithium arsenite (LiAsO2) which was allowed to react with HF or NH4F so as to obtain LiAsF6. Alternatively, arsenous trioxide (As2O3) is mixed with LiOH or Li2O or Li2CO3 and NH4F is required proportions and the mix is heated in an electric furnace in between temperatures 150-300xc2x0 C. for 4 hours continuously. The resultant product was examined by X-ray for purity and identity.
Lithium hexafluoro arsenate (LiAsF6) prepared by other methods suffer from the following disadvantages.
1. Toxic chemicals are required for preparation.
2. Difficult to handle toxic chemicals.
3. The product contains unreacted reactants as impurity
4. Partial reactions occur.
5. Needs several equipment to avoid pollution problems.
6. It is not based as solid state reactions.
The main object of this present invention is to provide a novel method for the preparation of lithium hexafluoro arsenate (LiAsF6) which obviates the draw backs mentioned above.
Another object of this present invention is to provide a complete solid state single/double stage preparation or partial solid state method of preparation.
Still yet another object of this invention is to avoid costly equipments to handle obnoxious gases used for the preparation of lithium hexafluoro arsenate (LiAsF6).
Yet still another object of this invention is to develop a single thermal procedure for the preparation of lithium hexafluoro arsenate (LiAsF6) or an intermediate lithium hypoarsenite (LiAsO2).
Another object of this present invention is to get the product without any side reaction or partial reaction.
Accordingly the present invention provides a process for the preparation of lithium hexafluoro arsenate comprising contacting arsenous trioxide with a lithium source and a fluorine source, all being in solid state and heating to obtain lithium hexafluoro arsenate.
In one embodiment of the invention, the lithium source is selected from the group consisting of LiOH, Li2O, LiNO3 and Li2CO3.
In another embodiment of the invention, the fluorine source is selected from HF and NH4F.
In one embodiment of the invention, the process comprises two stages comprising mixing the lithium source with arsenous trioxide in equimolar proportions and heating the slowly in an electric furnace initially to 100xc2x0 C. and then continuously for 4 hours at a temperature in the range of 200-300xc2x0 C. to obtain lithium meta arsenate (LiAsO2), reacting the lithium metaarsenite product with a fluorine source for about 4 hours to obtain lithium hexafluoro arsenate.
In a further embodiment of the invention, the fluorine source is selected from 10-30% HF and 6-9 times the molecular weight of NH4F.
In another embodiment of the invention, lithium meta arsenate (LiAsO2) is mixed with ammonium fluoride and heated in an electric furnace in between temperatures of 150-300xc2x0 C. continuously for 4 hours to obtain LiAsF3 confirmed by X-ray analysis.
In another embodiment of the invention, the reaction is carried out in a single step process comprising mixing a lithium source selected from the group consisting of LiOH, Li2O, Li2CO3 and LiNO3 with equimolar quantity of arsenous trioxide (As2O3) and seven times molecular weight ammonium fluoride, heating the solid mixture in an electric furnace at a temperature in the range of 150-300xc2x0 C. for 4 hours to obtain the desired product.
In one embodiment of the invention, the method comprises a full solid state thermal reaction of lithium salt with arsenous trioxide (As2O3) and ammonium fluoride mixture in the ratio 1:1:6-9 heated in an electric furnace continuously for 4 hours at a temperature in the range of 150-300xc2x0 C. to obtain lithium tetrafluoro arsenate confirmed by X-ray analysis.
In an embodiment of the invention LiOH or Li2O or Li2CO3 or LiNO3 is mixed with arsenous trioxide in the ratio 1:1 or get LiAsO2.
In another embodiment of the invention LiAsO2 is mixed with NH4F in a ratio 1:6-9.
In yet another embodiment of this invention LiAsO2 is mixed with 10-30% HF.
In yet another embodiment of this invention lithium salt, arseneous trioxide and NH4F are mixed in the ratio 1:1:6-9.
In still yet another embodiment of this invention an electric furnace is used to prepare LiAsO2 intermediate or the final product LiAsF6.
In another embodiment of this reaction all solid state or partial solid sate reaction is possible depending on the reactions envisaged for getting the end product LiASF2.